This invention relates in general to the field of band-reject filters, and in particular to microwave band-reject filters in communications receivers.
Band-reject filters are important in applications such as image noise suppression, suppression of image and local oscillator (LO) signals in mixers, suppression of adjacent channel interference in multi-channel communications systems, and rejection of noise caused by nearby synchronous hardware. While traditional band-reject filters are known, a need exists for a small, low cost, and light weight microwave band-reject filter suitable for the IRIDIUM.TM. satellite cellular communications system.
One example of where a band-reject filter might be used in a communication system is for receiver image noise suppression. A well-known occurrence in superheterodyne receivers is that the front end low-noise amplifier in such systems will generate thermal noise at the image frequency and that during the downconversion process the image noise will "fold over" onto the thermal noise at the desired receiver frequency. To avoid the associated degradation in system sensitivity, 15-20 decibels (dB) of image noise rejection is required prior to downconversion.
There are two general methods for providing such image rejection in communications receivers. The first uses a bandpass filter (image filter) centered at the desired receive frequency and connected between a low noise amplifier and a downconversion mixer. The bandpass filter is designed to provide 15-20 dB of noise suppression at the image frequency while passing the desired receive frequency (RF). For receiver applications where the intermediate frequency (IF) is very low relative to the RF frequency, the required Q of the image filter can be very high since the percentage difference between the RF and the image frequencies is very small (i.e., the local oscillator (LO) frequency is very close to the RF). High Q filters are typically realized using air dielectric cavity filter configurations. Major drawbacks to this method are that cavity filters are physically large, must be aligned prior to installation into a module, and require input/output transitions between the cavity transmission medium (coaxial, waveguide, etc.) and the planar transmission medium (typically microstrip).
The second method for providing image rejection incorporates a conventional image reject mixer whose topology is designed to downconvert the LO frequency plus the IF and the LO frequency minus the IF sidebands into separate IF output ports. However, considerable mixer complexity and development risk results from this method, especially at the higher microwave frequencies. The mixers must be well matched and the phase relationships well maintained in order to achieve adequate image suppression. In addition, the required local oscillator power for this method is 3 dB higher than that required for a comparable non-image rejection mixer.
Thus, what is needed is a relatively simple, efficient, low-cost, and easily maintained method and apparatus for image suppression in communications receivers in particular, and for band-reject filtering in general. It would be additionally desirable if such a method and apparatus would provide cheap, producible compatibility with most microwave and millimeter wave circuits. It would also be desirable if such a method and apparatus would include a symmetric configuration of traveling wave resonators for maintaining a close-matched passband characteristic impedance in the filter region (with less passband insertion loss) and enhanced coupling and electric field cancellation.